1. Field of the Invention
This invention relates to a radio wave absorbing material. More particularly, it relates to a radio wave absorbing material with high performance, capable of absorbing and attenuating radio waves such as VHF, UHF, microwaves, radar waves, semi millimeter waves, and millimiter waves over a wide band.
2. Description of the prior Art
Radio wave absorbing materials have been hitherto comprised of a ferrite sinter or ferrite powder, a semiconductor carbon fiber or powder, or a metallic fiber, dispersed in a rubber or resin, in a foamable styrol or urethane, or in an nonwoven fabric. Recently, as Japanese Laid-open Patent Application No. 62-123799 discloses, also proposed is a radio wave absorbing material comprising an electrically insulating potassium titanium short fiber whose surface is reduced or coated to make it semiconductive (10.sup.-2 to 10.sup.2 .OMEGA..cm; pressed powder state; 100 kg/cm.sup.2 which is compounded into a resin. This, however, can not necessarily give a satisfactory performance in respect of the radio wave absorption.
As Japanese Laid-open Patent Application No. 63-12198 discloses, further proposed is a radio wave absorbing electromagnetic shielding material comprising a ferromagnetic powder whose particle surfaces are coated with a conductive material having a conductivity of from 10.sup.-2 to 10.sup.2 .OMEGA..cm. This material, however, rather lays emphasis on electromagnetic shielding, with a lower radio wave absorbing performance, and thus necessarily results in a radio wave absorbing material with a narrow band.
Namely, the conventional radio wave absorbing materials have been greatly problemsome as having the following various problems.
Such problems include the problem concerning the directionality in radio wave absorption with respect to the direction of incidence of radio waves or the polarized waves, the problem of absorption performance, and the problem in wide-band absorption. There are additional problems that the radio wave absorbing performance is greatly affected by weathering resistance or stability of characteristics when radio wave absorbing materials are actually designed, manufactured and put into practical use, by adhesion to reflection plates (or materials) or the like or flexibility of radio wave absorbing materials when they are worked, or fur&her by deformation such as warpage when they are actually used, and that radio wave absorbing components accelerate the wear of machines when the radio wave absorbing materials are prepared using a kneader, a stirrer, a molding machine-or the like.
Thus, it has been strongly sought to provide a radio wave absorbing material that can eliminate or solve these many problems.